Memory devices have a wide range of uses in electronics and electronic devices. In general, various types of electronic memory exist, including hard disc memory, floppy disc memory, magnetic tape memory, optical disk memory, and so on. One of the more innovative and diversified types of memory is semiconductor memory.
Semiconductor memory operates on the basis of one or more memory transistors. Memory transistors are typically formed in a semiconductor substrate that is moderately to non-electrically conductive, but that can become conductive, or increasingly conductive, when a suitable voltage potential is applied to the transistor. Although various types of memory transistors exist, common memory transistors can store an amount of voltage or charge applied to the transistor, where different amounts of voltage or charge are equated to one (or more) bit(s) of digital information. By measuring electrical conductivity of such a transistor, a state of these bit(s) can be extracted at a later time. In this manner, digital information stored (as an amount of charge or voltage) within the transistor can be read and utilized for data processing. This forms one of the basic mechanisms that computers and related computing devices store and utilize electronic information.
As research and development in semiconductor science progresses, different forms and classifications of semiconductor memory have come to market. One general classification for semiconductor memory is volatile and non-volatile memory. Volatile memory generally requires application of an external voltage to a memory device to maintain a stored charge, or programmed bit. If the external voltage drops below a required level, the stored charge is lost. For a volatile memory device, such as random access memory (RAM), the lost charge results in lost data. Although volatile semiconductor memory has significant advantages, including high program and read speeds, the threat of data loss has made volatile semiconductor memory suitable primarily for RAM applications, especially given non-volatile mass storage alternatives such as hard drives, disc drives, and so on. Non-volatile memory, in contrast, can maintain stored charge without application of an external voltage source. Accordingly, non-volatile memory is not generally subject to data loss in response to power outage, or like occurrences. Non-volatile memory is therefore often utilized for applications in which resistance to power loss is important.
Static random access memory (SRAM) is one type of semiconductor memory generally classified as volatile memory. SRAM is a semiconductor technology that utilizes bi-stable latching circuitry to store a bit. The term “static” differentiates SRAM from dynamic random access memory (DRAM), which must be refreshed. The latching circuit employed by SRAM generally involves multiple transistors. For instance, a six transistor device (6T SRAM) is a common SRAM that stores a bit on four transistors that form two cross-coupled inverters that form a storage cell. The storage cell has two stable states used to denote a 0 or 1 of digital information. Two additional access transistors serve to control access to the storage cell during read and write operations. Other examples of SRAM configuration include 8T, 10T or more transistors per bit.
Generally, the fewer transistors employed for an SRAM cell reduces the size of a cell. The cost of processing a silicon memory device is fixed, in many aspects, and thus employing smaller cells and packing more bits on a wafer can reduce the cost per bit of memory. Cost and size are two factors important to memory design. Other factors include memory stability, read and write times, and so on. Ongoing research in memory technology continues to provide improvements in semiconductor memory technology, leading to more cost effective, and stable digital storage devices.